It is known to configure a shift register so that it produces a pseudo-random state sequence. Linear feedback shift registers configured thus are useful in a variety of contexts, particularly built in self test arrangement for very large scale integrated circuits. A linear feedback shift register can normally configured both as a signature analyser which will analyse the response of a logic block by compacting the outputs of the block; the content of the linear feedback shift register employed in this manner is called the signature of the block. The linear feedback shift register may also be configured as a pattern generator which will generate a pseudo-random pattern which may be used to stimulate the inputs of a logic block during a test mode. The present invention is particularly concerned with the use of a linear feedback shift register in the latter mode.
As is explained more particularly in detail hereinafter, it is known to configure the linear feedback shift register for the production of a pseudo-random pattern by the provision of a feedback connection from the last stage in the shift register to the input of the first stage and also to provide between at least one pair of adjacent stages an exclusive-OR gate which receives the output of the previous stage and the feedback signal, the output of the gate being connected to the input of the next stage.
For a variety of purposes it is desirable to be able to generate a pseudo-random sequence and also the reverse of that pseudo-random sequence. It is known to use an up/down counter to generate a state sequence. By definition such counters are reversible and it is straightforward and well known to generate a forward sequence and a reverse sequence in this manner. However, up/down counters will not produce a pseudo-random sequence and the use of an up/down counter to provide a state sequence is unnecessarily cumbersome.
It is therefore the object of the invention to provide a linear feedback shift register which is capable of generating a pseudo-random state sequence and also the reverse of that sequence.
It is a more particular object of the invention to provide a linear feedback shift register which can generate a pseudo-random state sequence and which is reversible at any stage of the sequence.
It is a further object of the invention to provide a linear feedback shift register of the foregoing type and readily capable of being automatically designed, for example as part of a very large scale integrated circuit which is designed by a process of automatic synthesis.
The basis of the present invention lies in the provision of two sets of connections which are alternatively selectable and multiplexing for the selection of one or other of the sets of connections. In some configurations, multiplexing is only employed for some of the stages of the register but in a preferred configuration multiplexing is provided for each stage in the register and with such a configuration it is feasible to provide a structure which will produce a pseudo-random sequence and also be reversible at any stage in the sequence.
The invention is applicable to both `standard ` linear feedback shift registers and `modular ` shift registers. For convenience, the invention will be generally explained with reference to `modular ` shift registers except where specifically mentioned otherwise.
Other objects and advantages will be apparent from the following description which is given by way of example with reference to the drawings.